In recent years, in the field of information recording and reproducing apparatuses, developments have been made to realize a still higher density and a narrower track width. In the case of reproducing information from tracks of a narrower width; however, adverse effects from a tracking error, i.e., the center of a signal reproducing element deviates from the center of a track, become noticeable, and a sufficient reproducing output cannot be ensured. The possible cause of this tracking error may be an undesirable shape of tracks formed on the recording medium.
For example, in a rotary head type magnetic tape device, the center line of the track width is desirably linear in shape. In practice; however, the center line is deformed from the linear line due to an eccentricity of a rotary drum during a signal recording operation, a running state of a magnetic tape and a linearity of a lead of a fixed drum, etc. In the case of a magnetic disk device of a sector servo system, on-disk position indicative information is recorded by a servo track writer. However, due to oscillation of the servo track writer, or a disk, etc., the position indicative information recorded on the disk does not draw an accurate circle. Therefore, also the center line of the track recorded based on the position indicative information is displaced from the circle. Furthermore, in the case of a disk device adopting a disk in which a magnetic signal or an optical signal is pre-formatted, the center line of the track width has an acentric component generated when mounting the disk to the device. Hereinafter, the deviation of the center line of the track width with respect to a desirable shape is referred to as a track bend irrespectively of the kind and the shape of the recording medium.
In general, the track bend has a basic frequency f.sub.tr, and this frequency component is the most important factor. For example, in the case of the rotary head type magnetic tape device, the basic frequency f.sub.tr corresponds to such frequency that a time interval from an approach of the magnetic head to the magnetic tape to a retrieval thereof is one period. While in the disk device adopting the pre-formatted disk, the basic frequency corresponds to the rotation frequency of the disk.
As an example of the track bend, FIG. 47 shows a track recorded by the rotary head type magnetic tape device. As shown by an alternate long and short dashed line in the figure, the center line of the track is wounded and is deformed from the desirable shape shown by the solid line. Therefore, the track bend Tr in a track scanning period of from t1 till t5 of the magnetic head is as shown in FIG. 48.
As the track becomes narrower, the permissible range of the tracking error is reduced. Therefore, in order to ensure a requested tracking precision, the dynamic tracking method has been adopted, wherein the signal reproducing element is mounted to an actuator which can be deformed in a track widthwise direction, to make the signal reproducing element follow the track bend.
As shown in the control block diagram of FIG. 49, in the conventional information recording and reproducing apparatus adopting the dynamic tracking technique, an error detector 101 detects a relative position error between the center line of a signal reproducing element 102 and the center line of the track based on a reproducing signal of the signal reproducing element 102 and outputs it as an error signal Err.
The error detector 101 is equivalently expressed by a comparator 101a for comparing the position X of the signal reproducing element 102 with the track bend Tr and an error detection gain 101b for converting a relative position error Xerr into a voltage level. In practice, the error detector 101 is not only arranged so as to include circuits 101a and 101b but also arranged so as to directly detect the error signal Err based on the reproducing signal of the signal reproducing element 102, for example, using the sector servo system in the disk device. Therefore, the described error detector 101 has such a drawback that the track Tr and the position X cannot be detected independently.
Furthermore, a drive circuit 103 applies an operation signal Sd1 to an actuator 104 based on the error signal Err, and the actuator 104 moves the signal reproducing element 102 in a track widthwise direction. As a result, the signal reproducing element 102 is guided to a position where the center line coincides with the control line of the track.
Especially, in the case of the rotary head type magnetic tape device, a magnetic tape is wound around a rotary drum at a predetermined winding angle, and a scanning period in which the magnetic head scans on the magnetic tape and a non-scanning period in which the magnetic head does not scan on the magnetic tape are formed. In this case, as shown in FIG. 50, a switch 105 is provided between the error detector 101 and the drive circuit 103 shown in FIG. 49. By the switch, the magnetic head is switched between a scanning state and a non-scanning state, and a signal of 0 level is inputted to the drive circuit 103 in the non-scanning period.
In respective arrangements shown in FIG. 49 and FIG. 50, the scanning period is used in common. Thus, hereinafter, explanations will be given mainly through the information recording and reproducing apparatus shown in FIG. 49. The control system of the information recording and reproducing device is, for example, shown by a block of a transfer function shown in FIG. 51.
Therefore, the open-loop transfer function G.sub.open of the control system is given by the equation (1): EQU G.sub.open =K.sub.e .multidot.G.sub.1 (s).multidot.G.sub.A (s) (1),
wherein s is a Laplacean operator. In general, the actuator 104 is a second-order lag element having a resonance point, and the transfer function depends on a frequency as expressed in a function G.sub.A (s).
Normally, in the control system adopting the actuator 104 having a resonance point, effects from the resonance point are removed by means of, for example, a notch filter, etc., and a control band (gain-crossover frequency of open-loop transfer characteristics) is generally set lower than the resonance frequency f.sub.r.
However, updated information recording and reproducing apparatuses are required to have the following features: The fundamental frequency f.sub.tr of the track bend Tr is sufficiently high, and in the case of controlling a dynamic tracking, the signal reproducing element 102 follows with high precision even with respect to the fundamental frequency component f.sub.tr and a high-order component. It is therefore required to set the resonance frequency f.sub.r to be very high. However, when the resonance frequency f.sub.r is raised by setting a spring constant of the actuator 104 high, a power consumption is increased. Thus, it is difficult to apply the described arrangement in practical applications.
Thus, it is a necessary condition that the resonance frequency f.sub.r of the actuator 104 is slightly higher than the fundamental frequency f.sub.tr of the track bend Tr. As a result, a sufficient control band cannot be obtained when removing the effects from the resonance point by a notch filter, etc.
Therefore, in order to solve the above problem, a control system in which the control band is set higher than the resonance frequency f.sub.r of the actuator 104 is known, and the control system adopting the control system is used. Hereinafter, the described control system is referred to as a punch-through servo system. In the control system, as shown in FIG. 52, in a vector trace of the open-loop transfer function, a gain achieved when the phase is in a vicinity of -180.degree. is set sufficiently high. Furthermore, in the subsequent vector trace, each element which constitutes a control system is set in such a manner that the gain is converged to 0 while observing -1 to the left. In the field of a control engineering, the condition for allowing the vector trace of the open-loop transfer function to pass while observing -1 to the left side is a stable condition, and thus, it can be said that the punch-through servo system is an effective control system which permits a control over a wide band while maintaining the stability.
The control system of FIG. 51 adopts a punch-through servo system in which a gain, a phase compensation circuit, and a constant of the low pass filter which constitute the drive circuit 103 are adjusted. Such low pass filter is adopted to remove the effects from the resonance point of the higher-order of the actuator 104. In this case, the open-loop transfer function G.sub.open of the control system can be derived from the equation (1).
An example of the open-loop transfer characteristics G.sub.open of the control system adopting the punch-through servo system is shown in FIG. 53. The control system satisfies the stable conditions, and the gain-crossover frequency f.sub.c is set substantially higher than the resonance frequency f.sub.r of the actuator 104, thereby permitting the signal reproducing element 102 to follow the high-order component of the track bend Tr.
However, in the control system adopting the punch-through servo system, the follow-up frequency band with respect to the track bend Tr is expanded, while the open-loop gain in the fundamental frequency f.sub.tr of the track bend Tr is limited due to the phase compensation for satisfying the conditions for the punch-through servo system. Namely, the follow-up precision of the signal reproducing element 102 with respect to the fundamental frequency f.sub.tr of the track bend Tr cannot be improved.
As the fundamental frequency component f.sub.tr is the most important factor for the track bend Tr, especially, in the case of a narrow track width, it is difficult to ensure a sufficient reproducing power, and meet the specification.
In consideration of the above, it is desired to improve the open-loop gain while maintaining a frequency band that can be controlled. In the punch-through servo system, in a higher frequency band than a resonance frequency f.sub.r of the actuator 104, a minimum phase margin is ensured by utilizing effects of the phase-lead compensation. Therefore, any mean to improve the open-loop gain to solve the above problem would not lead a reduction in the phase margin.
In the information recording and reproducing device, for example, an error detector 101 of the rotary head type magnetic tape device adopts a two frequency pilot signal system, a four pilot signal system, a wobbling system etc., as a detection method of an error. While in the disk device, a sector servo system, a magnetic signal pre-format servo systems an optical signal pre-format servo system, etc., is adopted. In any of the described error detection methods, an error signal Err indicative of a relative position error between the signal reproducing element 102 and a track as desired is directly detected. Therefore, the error detector 101 has such a drawback that the track bend Tr cannot be detected alone by isolating therefrom the displacement X of the signal reproducing element 102. Namely, the method of improving the follow-up precision by feed-forwarding the track bend Tr, i.e., the target to be followed by the signal reproducing element 102 cannot be adopted.
In order to solve the described problem, for example, Japanese Unexamined Patent Application No. 71415/1991 (Tokukaihei 3-71415) discloses a rotary head type magnetic tape device including a neuron learning control part in the control loop. According to the neuron learning control system adopted in the magnetic tape device, when a desired value has a periodically repetitive waveform, the offset can be made significantly smaller. The respective track bends Tr of respective tracks of the magnetic tape are strongly interrelated, and by performing the neuron learning control, the magnetic head in the magnetic tape device can follow the track bend Tr at high precision.
However, the neuron learning control part is constituted by the positive feedback loop containing an dead time element, and a uncountable number of poles exist therein. As a result, when introducing the neuron learning control part, the control system is apt to become unstable compared with the state before the neuron learning control element is adopted. On the other hand, in the control system adopting the punch-through servo system, as only a minimum phase margin is formed, it is difficult to introduce therein the neuron learning control part. Furthermore, in the neuron control, the problem that a residual eccentricity is increased on the contrary with respect to a desired value having a different frequency from the repetitive frequency is newly raised. Thus, the introduction of the described neuron learning control part do not offer the solution to the above-mentioned problem.
As another method of solving the described problem, Japanese Unexamined Patent Application No. 114781/1995 (Tokukaihei 7-114781) discloses a method of measuring the track bend Tr to be stored and feeding it forward when reproducing a signal. However, as this method raises a new problem that a special mechanism and time for measuring the track bend Tr are required.
Furthermore, although it is not a solution for directly solving the described problem, Japanese Unexamined Patent Application No. 52563/1994 (Tokukaihei 6-52563) discloses an information recording and reproducing apparatus in which a control ability is improved by adopting a disturbance observer. As shown in FIG. 54, the information recording and reproducing apparatus includes a disturbance observer 106 in addition to the arrangement of the information recording and reproducing apparatus shown in FIG. 49. In this figure, D indicates a disturbance to be exerted on the actuator 104, and 106b indicates a position detector for detecting the position X of the signal reproducing element 102.
In the disturbance observer 106, an actuator simulation circuit 106a electrically simulates characteristics of the actuator 104, and the position X of the signal reproducing element 102 is estimated based on the drive signal Sd to be applied to the actuator 104. On the other hand, the position detector 106b detects a position X of the signal reproducing element 102. The comparator 106c estimates a disturbance D by comparing an output of the actuator simulation circuit 106a with an output of the position detector 106b. The estimated disturbance D is fed forward to a drive signal through an inverse characteristic simulation circuit 106d which simulates the inverse characteristics of the actuator 104. As a result, the disturbance observer 106 cancels out the disturbance D applied to the actuator 104.
Therefore, like the conventional information recording and reproducing apparatus, in the case where the effect of the track bend Tr is relatively small as the track width is relatively wide, and a main cause of hindering the dynamic tracking control is a disturbance, i.e., the control is considered to a constant value control, the disturbance observer 106 is very effective.
However, the updated information recording and reproducing apparatus which have developed in terms of higher density (narrower track), it is required to make the signal reproducing element 102 follow over a wide band with high precision and over wide band even with respect to a fine track bend Tr which has not been a problem in the conventional arrangement. However, the disturbance observer 106 is provided only for suppressing the disturbance D which is exerted onto the actuator 104, and it does not improve a follow-up function with respect to the desired value. Therefore, in the described case, the disturbance observer 106 does not offer desirable effects.
As described, any of the conventional techniques does not offer a solution to achieve an information recording and reproducing apparatus which permits only a relative position error between the track and the signal reproducing element 102 to be detected by the error detector 101, and the signal reproducing element 102 to follow the track bend Tr over a wide band with high precision. This is a serious drawback in the control system adopting the punch-through servo system.